This invention relates generally to rotation sensors and particularly to ring laser gyroscope rotation sensors. Still more particularly, this invention relates to apparatus and methods for reducing the random walk error of a ring laser gyroscope caused by the tendency of the counterpropagating beams of ring laser gyroscopes to lock to a common frequency at low rotation rates.
A ring laser gyroscope employs the Sagnac effect to detect rotation. Two counter propagating light beams in a planar closed loop will have transit times that differ in direct proportion to the rotation rate of the loop about an axis perpendicular to the plane of the loop. The loop need not be planar, but the planar ring laser gyroscope has the simplest type of optical path.
There are in general two basic techniques for utilizing the Sagnac effect to detect rotations. A first technique is the interferometric approach, which involves measuring the differential phase shift between two counterpropagating means injected from an external source, typically a laser, into a Sagnac ring. The ring may be defined by mirrors that direct the light beams around the path or by a coil of optical fiber. Beams exiting the path interfere and create a pattern of light and dark lines that is usually called a fringe pattern. Absolute changes in the fringe pattern are indicative of rotation of the ring. The primary difficulty with such devices is that the changes are very small for rotation rates of interest in guidance applications.
The ring laser gyroscope uses the resonant properties of a closed cavity to convert the Sagnac phase difference between the counter propagating beams into a frequency difference. The high optical frequencies of about 10.sup.15 Hz for light used in ring laser gyroscopes cause the minute phase changes to become beat frequencies that are readily measured.
A ring laser gyroscope has a sensor axis that passes through the closed paths traversed by the counterpropagating beams. When the ring laser gyroscope is not rotating about its sensor axis, the optical paths for the two counterpropagating beams have identical lengths so that the two beams have identical frequencies. Rotation of the ring laser gyroscope about its sensor axis causes the effective path length for light traveling in the direction of rotation to increase while the effective path length for the wave traveling opposite in direction to the rotation decreases.
Ring laser gyroscopes may be classified as passive or active, depending upon whether the lasing, or gain, medium is external or internal to the cavity. In the active ring laser gyroscope the cavity defined by the closed optical path becomes an oscillator, and output beams from the two directions can be combined to beat together to give a beat frequency that is a measure of the rotation rate. The oscillator approach means that the frequency filtering properties of the cavity resonator are narrowed by many oders of magnitude below the passive cavity and give very precise rotation sensing potential. To date the major ring laser gyroscope rotation sensor effort has been put into the active ring laser. Presently all commercially available optical rotation sensors are active ring laser gyroscopes.
In the active ring laser gyroscope, the length of the closed optical path is controlled by means of at least one moveable mirror to maintain an intensity maxima. Maximum intensity is achieved when the total closed pathlength contains an integral number (about 10.sup.6) of the wavelength for which the lasing gain is a maximum. Intensity maxima are found separated by a change of pathlength by one wavelength. The number of wavelengths of the pathlength is referred to as the mode of the laser gyroscope.
When the rotation rate of the ring laser gyroscope is within a certain range, the frequency difference between the beams disappears. This phenomenon is called frequency lock-in, or mode locking, and is a major difficulty with the ring laser gyroscope because at low rotation rates the ring laser gyroscope produces a false indication that the device is not rotating. If the rotation rate of a ring laser gyroscope starts at a value above that where lock-in occurs and is then decreased, the frequency difference between the beams disappears at a certain input rotation rate. This input rotation rate is called the lock-in threshold and may be denoted .OMEGA..sub.L. The range of rotation rates over which lock-in occurs is the deadband of the ring laser gyroscope.
Lock-in is believed to arise from coupling of light between the beams. The coupling results primarily from backscatter off the mirrors that confine the beams to the closed path. Backscatter causes the beam in each direction to include a small component having the frequency of the beam propagating in the other direction. The lock-in effect in a ring laser gyroscope is similar to the coupling that has been long been observed and understood in conventional electronic oscillators.
In addition to causing erroneous rotation rate information to be output from a ring laser gyroscope, lock-in causes standing waves to appear on the mirror surfaces. These standing waves may create a grating of high and low absorption regions, which create localized losses that increase the coupling and the lock-in. The mirrors may be permanently distorted by leaving a ring laser gyroscope operating in a lock-in condition.
Any inability to accurately measure low rotation rates reduces the effectiveness of a ring laser gyroscope in navigational systems. There has been substantial amount of research and development work to reduce or eliminate the effects of lock-in to enhance the effective use of ring laser gyroscopes in such systems.
There are several known approaches to solving the problems of lock-in. These approaches take the form of biasing the frequency difference between the counter rotating beams such that the lock-in region is avoided for either all or the greatest part of the operating time of the laser gyroscope. Electro-optical means, such as a Faraday cell or magnetic mirror, may be employed to bias the frequencies; or the laser gyroscope may be biased by the gyro body. Typical practice is to periodically reverse the rate in either the electrical or mechanical methods, since the applied bias is often not known to the accuracy required to permit inertial navigation. Rate reversals permit the bias applied to be averaged out.
The most common successful approach involves mechanically oscillating the ring laser gyroscope about its sensor axis so that the device is constantly sweeping through the deadband and is never completely locked therein. This mechanical oscillation of the ring laser gyroscope is usually called dithering. A typical ring laser gyroscope may be dithered at about 400 Hz with an angular displacement of a few arc minutes.
Dithering is accomplished by mounting the ring laser gyroscope frame on a flexure device that includes a plurality of vanes or blades extending from a central portion. Each blade has a pair of piezoelectric elements mounted on opposite sides thereof. Voltages are applied to the piezoelectric elements such that one piezoelectric element on each blade increases in length while the other piezoelectric element decreases in length. The effect of these length changes in the piezoelectric elements is transmitted to the blades through the mounting of the piezoelectric elements thereon. Increasing the length of one side of each blade while shortening the other side causes the blades to flex or bend so that each blade experiences a small rotation about the ring laser gyroscope axis. The voltage is oscillatory so that the blades are constantly vibrating in phase, and the ring laser gyroscope frame mounted to the blades rotates about the axis.
The amplitude of the dithering is generally controlled and monitored. Since the dither oscillation angular velocity and displacement relative to a support structure can be constantly monitored, they may be excluded from the output signal of the ring laser gyroscope. It has been found that a constant dithering amplitude is undesirable due to the residual lock-in error. Typical practice is to superimpose a random signal upon the amplitude of the dither driving amplifier.
Even with dithering there is a residual due lock-in. When the sign or direction of the frequency difference reverses, the two beams tend to lock-in since at some point the frequency difference therebetween is zero. Since the output angle of the ring laser gyroscope is generally derived from the frequency difference, an error accumulates in the output angle. The periods of time when the two beams are locked-in are usually very short so that the resulting output angle error is very small for any single sign change. Nevertheless, the error resulting from lock-in during signal reversal of the frequency difference is cumulative, and in time may become significant, particularly in precision navigational systems. This error is sometimes called random walk or random drift.
A ring laser gyroscope may be mounted upon a gimballed system. Typically in a gimballed mounting system the sensing axes of the ring laser gyroscopes are held fixed relative to an inertial reference or relative to coordinates fixed upon the earth.
Another method for mounting ring laser gyroscopes is to attach them to a vehicle so that the sensor axes are aligned with a set of orthogonal axes on the vehicle. Accelerometers are also attached to the vehicle, and a computer transforms data from the accelerometers and the rotation sensors into navigation coordinates. This configuration is called a strapped down mechanization. Because of its scale factor accuracy and dynamic range, the ring laser gyroscope is more suitable for a strapped down system than a spinning rotor gyroscope. The scale factor accuracy of a ring laser gyroscope is typically five to ten times that of a spinning rotor gyroscope.
U.S. Pat. No. 4,115,004 to Hutchings et al., and assigned to Litton Systems, Inc., assignee of the present invention, discloses a dual spring system that mounts a counterweight to the ring laser gyroscope case to reduce oscillatory motion of the case due to oscillation of the gyroscope. This dual spring system includes a first set of springs mounted between the case and the gyroscope and a second set of springs mounted between the case and the counterweight.
U.S. Pat. No. 4,309,107 to McNair et al., and assigned to Litton systems, Inc., assignee of the present invention, discloses a ring laser gyroscope dither mechanism for isolating vibrational energy generated by dithering the gyroscope and prevents that energy from passing to the mounting case of the gyroscope. McNair et al. discloses a three spring system for mounting a gyroscope to a housing or case, mounting a counterweight to the gyroscope and mounting the counterweight to the case. This arrangement reduces the amount of angular vibrational energy that passes to the case of the gyroscope by using the counterweight to provide a reaction to the oscillations within the gyroscope caused by mechanically dithering to prevent lock-in.
U.S. Pat. No. 3,464,657 to Bullard discloses a single set of springs connected between the frame and mounting platform of an aircraft instrument to isolate vibrational energy from the instrument.
U.S. Pat. No. 3,373,650 to Killpatrick discloses a dithering system in which a varying bias in the frequency is applied to at least one of the counterpropagating beams. Killpatrick discloses a Faraday cell and two quarter wave plates in the path of the counterpropagating light beams. The Faraday cell includes a coil that is energized by an oscillatory current to produce an oscillatory magnetic field that interacts with the counterpropagating beams. The varying bias causes a varying frequency difference between the counterpropagating beams. This frequency difference is generally greater than the frequency difference that occurs at the lock-in threshold. The polarity of the frequency difference is periodically alternated so that the time integral of the frequency difference over the time interval between sign reversals is substantially zero.
U.S. Pat. No. 3,467,472 discloses a dithering system similar to that disclosed by Killpatrick in U.S. Pat. No. 3,373,650. However, that patent discloses randomly changing the amount of bias in order to reduce the random walk resulting from lock-in when the sign change of the bias reverses.
U.S. Pat. No. 4,248,534 issued Feb. 3, 1981 to Elbert discloses a mechanism that sinusoidally dithers a ring laser gyroscope. The output of the ring laser gyroscope is corrected for error caused by lock-in at the extremes of the oscillations. The outputs of a photodiode that measures the light intensity of the interference pattern at each zero dither velocity are accumulated. When the accumulated phase error becomes 2.pi., an overflow pulse is generated and added to the output of the ring laser gyroscope to correct for accumulated errors caused by lock-in at the zero dither velocity.
U.S. Pat. No. 4,526,469 to Egli et al. discloses a discriminant apparatus for ring laser gyroscopes. The discriminant is related to the weighted vector sum of the coupling of energy between the counterpropagating waves and can be used to indicate relative changes of the magnitude of the lock-in rate. The discriminant is used in a closed loop to alter the path traveled by the counterpropagating waves to adjust the weighted vector sum of the energy coupled therebetween so that the effects of lock-in are reduced.
U.S. Pat. No. 4,529,311 to Morgan et al. discloses the use of an incremental error parameter related to the instantaneous phase difference between the two counterpropagating beams in a ring laser angular rate sensor to generate a set of error parameters that correspond to the contribution of lock-in error in the output of the sensor. The error parameters can be used in a control loop for indirectly reducing the error in the sensor output or the error parameters can be used for a combination of error reduction and compensation.
U.S. patent application No. 448,363 filed Dec. 9, 1982 and assigned to Litton Systems, Inc., assignee of the present invention, discloses a dither controller for a ring laser gyroscope angular rotation sensing system in which a sensor produces a signal indicative of the dither angular motion. The drive circuit for dithering the ring laser gyroscope body is sampled at time intervals that are shorter than the period of the natural oscillation of the ring laser gyroscope body. The absolute values of the samples are averaged to obtain a measure proportional to the average of the peak amplitude of the dither oscillation signal. When the peak amplitude decays to a predetermined minimum value, a driving torque is applied to the ring laser gyroscope. When the sum of the samples increases to sufficiently, the ring laser gyroscope is allowed to oscillate at its natural frequency while the amplitude slowly decays to the minimum value at which the driving torque is applied again. By this means, the maximum dither input rate, and the dither depth, are controlled on average to the desired value.